The present invention relates to an inductive heating apparatus, and more particularly to an inductive heating apparatus with multiple high frequency energy sources for heating loads.
The conventional inductive heating apparatus, shown in U.S. Pat. No. 4,338,503, includes a pair of on-off switching means to operate a resonant circuit. Therefore, in the case of an inductive heating apparatus with multiple high frequency energy sources, the construction as taught in the above referenced patent is complicated and expensive because of the need for multiple pairs of switches. Thus, it is desirable to provide an induction heating apparatus which is simple and less costly and having fewer switches than in the prior art.
Recently, a desirable type of inductive heating apparatus is available, such as Toshiba Inductive Heater model MR-105 shown in Toshiba Review Vol. 38, No. 2, 1983.
This type of inductive heating apparatus, having an inverter of a single-end type, has a resonant circuit which includes an inductive heating coil a capacitor connected in series to the coil, and a switching means connected in parallel to the capacitor.
The resonant frequency is determined by the condition and size of the load which is inductively coupled the coil, because the resonant circuit resonates in series between the coil and the capacitor.
However, in the inductive heating apparatus with multiple high frequency energy sources of the above type, the resonant frequency may be different for the different sources. If the difference of frequency is larger than 3 KHz for example, there is a problem that noise sounds occurs when the multiple high frequency energy sources are operated at the same time.
A multiple coil prior art inductive heating apparatus is illustrated by U.S. Pat. No. 4,092,510. In this reference, however, multiple heating coils are supplied from a common source to suppress noise interference.